An electric machine or electric motor may feature a rotor with permanent magnets and a stator. For example, the electric machine may comprise a permanent magnet (PM) motor, an interior permanent magnet (IPM) motor, or IPM synchronous motor. The electric machine may have a rotor angle or rotor position that is difficult to estimate accurately with the application of conventional approaches.
There are several different conventional technical approaches for estimating an initial rotor position of an electric machine. Under a first approach, because the back-electromotive force (back-EMF) voltages of a motor are functions of the rotor position with respect to the stator, a sensor may extract rotor position information from the back-EMP waveform of an unexcited phase of a multi-phase alternating current motor. However, the first approach is not suitable for estimating the rotor position if the motor is operating at low rotational speeds because the back-EMF voltages are frequently too low to be accurately detected at low rotational speeds below a speed threshold (e.g., 100 revolutions per minute). In addition, electromagnetic noise or interference from switching transients in one or more current-carrying phases of the electric machine can lead to error in estimates of rotor position.
Under a second approach, a computer estimates the rotor position based on phase voltages and phase currents of the electric machine. For example, the computer may be associated with a Kalman filter to estimate the rotor position. The second approach is often computationally intensive and susceptible to quantification errors, truncation errors, and measurement inaccuracies. The rotor position estimate of the second approach may be inaccurate when the motor is operating at low speeds below a speed threshold (e.g., 100 revolutions per minute (RPM)) because of the drift of integration functions, for example. To be sufficiently accurate, the second approach generally needs to compensate for changes in motor parameters associated with temperature change, which contributes to the complexity of the computations.
Under a third approach, the rotor saliency in permanent magnet motors is used to estimate rotor position based on the asymmetrical rotor structure or magnetic saturation. The third approach may use the injection of a high frequency test signal into control terminals of the motor. Although the third approach works at low rotational speeds of the motor and is generally independent of rotor speed, the third technique cannot be used with non-salient permanent magnet motors, such surface mount, permanent magnet alternating current motors without rotor saliency. Thus, there is a need for an improved method and system for estimating rotor angle or rotor position of an electric machine to address one or more of the above noted deficiencies.